APPLICATION NOTE NO. 27Druck

Minimizing Strain Gauge Pressure Sensor Errors
July 2005

Application note in pdf format

Note: See Application Note 27 for older production (Paine) strain gauge pressure sensors.

The following Sea-Bird instruments use strain gauge pressure sensors manufactured by GE Druck:

• SBE 16plus and 16plus-IM SEACAT (not 16 *) with optional strain gauge pressure sensor

• SBE 19plus SEACAT Profiler (not 19 *)

• SBE 25 SEALOGGER CTD, which uses SBE 29 Strain-Gauge Pressure Sensor (built after March 2001)

• SBE 26plus SEAGAUGE Wave and Tide Recorder with optional strain gauge pressure sensor in place of Quartz pressure sensor

• SBE 37 MicroCAT (37-IM, -IMP, -SM, -SMP, -SI, and -SIP) with optional pressure sensor (built after September 2000)

• SBE 39 Temperature Recorder with optional pressure sensor (built after September 2000) and SBE 39-IM Temperature Recorder with optional pressure sensor

• SBE 49 FastCAT CTD Sensor

• SBE 50 Digital Oceanographic Pressure Sensor

• SBE 52-MP Moored Profiler CTD and Optional DO Sensor

* Note: SBE 16 and SBE 19 SEACATs were originally supplied with other types of pressure sensors. However, a few of these instruments have been retrofitted with Druck sensors.

The Druck sensors are designed to respond to pressure in nominal ranges 0 - 20 meters, 0 - 100 meters, 0 - 350 meters, 0 - 600 meters, 0 – 1000 meters, 0 – 2000 meters, 0 – 3500 meters, and 0 – 7000 meters (with pressures expressed in meters of deployment depth capability). The sensors offer an initial accuracy of 0.1% of full scale range.

The term psia means pounds per square inch, absolute (absolute means that the indicated pressure is referenced to a vacuum).

For oceanographic purposes, pressure is most often expressed in decibars (1 dbar = 1.4503774 psi). A dbar is 0.1 bar; a bar is approximately equal to a standard atmosphere (1 atmosphere = 1.01325 bar). For historical reasons, pressure at the water surface (rather than absolute or total pressure) is treated as the reference pressure (0 dbar); this is the value required by the UNESCO formulas for computation of salinity, density, and other derived variables.

Some oceanographers express pressure in Newtons/meter² or Pascals (the accepted SI unit). A Pascal is a very small unit (1 psi = 6894.757 Pascals), so the mega-Pascal (MPa = 10⁶ Pascals) is frequently substituted (1 MPa = 100 dbar).

Since the pressure sensors used in Sea-Bird instruments are absolute types, their raw data inherently indicate atmospheric pressure (about 14.7 psi) when in air at sea level. Sea-Bird outputs pressure in one of the following ways:

• CTDs that output raw data (SBE 16plus, 16plus-IM, 19plus, 25, and 49) and are supported by SEASOFT’s SEASAVE (real-time data acquisition) and SBE Data Processing (data processing) software – In SEASOFT, user selects pressure output in psi (not psia) or dbar. SEASOFT subtracts 14.7 psi from the raw absolute reading and outputs the remainder as psi or converts the remainder to dbar.
• SBE 26plus – Real-time wave and tide data is output in psia. Wave and tide data stored in memory is processed using SEASOFT for Waves’ Convert Hex module, and output in psia. Tide data can be converted to psi by subtracting a barometric pressure file using SEASOFT for Waves’ Merge Barometric Pressure module.
• SBE 50 – User selects pressure output in psia (including atmospheric pressure) or dbar. Calculation of dbar is as described above.
• For all other instruments that can output converted data in engineering units (SBE 16plus, 16plus-IM, 19plus, 37, 39, 39-IM, 49, and 52-MP) – Instrument subtracts 14.7 psi from the raw absolute reading and converts the remainder to dbar.

Note: SBE 16plus, 16plus-IM, 19plus, 49, and 52-MP can output raw or converted data.

Despite the common nomenclature (CTD = Conductivity - Temperature - Depth), all CTD instruments measure pressure, which is not quite the same thing as depth. The relationship between pressure and depth is a complex one involving water density and compressibility as well as the strength of the local gravity field, but it is convenient to think of a decibar as essentially equivalent to a meter, an approximation which is correct within 3% for almost all combinations of salinity, temperature, depth, and gravitational constant.

SEASOFT (most instruments)

SEASOFT offers two methods for estimating depth from pressure:

• For oceanic applications, salinity is presumed to be 35 PSU, temperature to be 0^o C, and the compressibility of the water (with its accompanying density variation) is taken into account. This is the method recommended in UNESCO Technical Paper No. 44 and is a logical approach in that by far the greatest part of the deep-ocean water column approximates these values of salinity and temperature. Since pressure is also proportional to gravity and the major variability in gravity depends on latitude, the user's latitude entry is used to estimate the magnitude of the local gravity field:

• SBE 16plus, 16plus-IM, 19plus, 25, and 49 -- User is prompted to enter latitude if Depth [salt water] is selected as a display variable in SEASAVE or as an output variable in the Data Conversion or Derive module of SBE Data Processing.
• SBE 37-SM, 37-SMP, 37-IM, and 37-IMP -- User is prompted to enter latitude if Depth [salt water] is selected as an output variable in the Derive module of SBE Data Processing.
• SBE 37-SI, 37-SIP, and 50 -- Latitude is entered in the instrument's EEPROM using the LATITUDE= command in SEASOFT's SEATERM (terminal program) software.
• SBE 39 and 39-IM -- User is prompted to enter latitude if conversion of pressure to depth is requested when converting an uploaded .asc file to a .cnv file in SEATERM.

• For fresh water applications, compressibility is not significant in the shallow depths encountered and is ignored, as is the latitude-dependent gravity variation. Fresh water density is presumed to be 1 gm/cm and depth (in meters) is calculated as 1.019716 * pressure (in dbars). No latitude entry is required for the following:

• SBE 16plus, 16plus-IM, 19plus, 25, and 49 -- If (Depth [fresh water] is selected as a display variable in SEASAVE or as an output variable in the Data Conversion or Derive module of SBE Data Processing).
• SBE 37-SM, 37-SMP, 37-IM, and 37-IMP -- If Depth [fresh water] is selected as an output variable in the Derive module of SBE Data Processing.

SEASOFT for Waves (SBE 26plus SEAGAUGE Wave and Tide Recorder)

SEASOFT for Waves’ Merge Barometric Pressure module subtracts a user-input barometric pressure file from the tide data file, and outputs the remainder as pressure in psi or as depth in meters. When converting to depth, the compressibility of the water is taken into account by prompting for user-input values for average density and gravity. See the SBE 26plus manual’s appendix for the formulas for conversion of pressure to depth.

Initial accuracy and resolution are expressed as a percentage of the full scale range for the pressure sensor. The initial accuracy is 0.1% of the full scale range. Resolution is 0.002% of full scale range, except for the SBE 25 (0.015% resolution). For best accuracy and resolution, select a pressure sensor full scale range to correspond to no more than the greatest depths to be encountered. The effect of this choice on CTD accuracy and resolution is tabulated below:

The meaning of accuracy, as it applies to these sensors, is that the indicated pressure will conform to true pressure to within ± maximum error (expressed as equivalent depth) throughout the sensor's operating range. Note that a 7000-meter sensor reading + 7 meters at the water surface is operating within its specifications; the same sensor would be expected to indicate 7000 meters ± 7 meters when at full depth.

Resolution is the magnitude of indicated increments of depth. For example, a 7000-meter sensor on an SBE 25 (resolution 1.05 meters) subjected to slowly increasing pressure will produce readings approximately following the sequence 0, 1.00, 2.00, 3.00 (meters). Resolution is limited by the design configuration of the CTD's A/D converter. For the SBE 25, this restricts the possible number of discrete pressure values for a given sample to somewhat less than 8192 (13 bits); an approximation of the ratio 1 : 7000 is the source of the SBE 25's 0.015% resolution specification.

Note: SEASOFT (and other CTD software) presents temperature, salinity, and other variables as a function of depth or pressure, so the CTD's pressure resolution limits the number of plotted data points in the profile. For example, an SBE 25 with a 7000-meter sensor might acquire several values of temperature and salinity during the time required to descend from 1- to 2-meters depth. However, all the temperature and salinity values will be graphed in clusters appearing at either 1 or 2 meters on the depth axis.

High-range sensors used in shallow water generally provide better accuracy than their absolute specifications indicate. With careful use, they may exhibit accuracy approaching their resolution limits. For example, a 3500-meter sensor has a nominal accuracy (irrespective of actual operating depth) of ± 3.5 meters. Most of the error, however, derives from variation over time and temperature of the sensor's offset, while little error occurs as a result of changing sensitivity.

Offset Errors

Note: Follow the procedures below for all instruments except the SBE 26plus (see the 26plus manual for details).

The primary offset error due to drift over time can be eliminated by comparing CTD readings in air before beginning the profile to readings from a barometer. Follow this procedure:

1. Allow the instrument to equilibrate in a reasonably constant temperature environment for at least 5 hours. Pressure sensors exhibit a transient change in their output in response to changes in their environmental temperature; allowing the instrument to equilibrate before starting will provide the most accurate calibration correction.
2. Place the instrument in the orientation it will have when deployed.
3. Set the pressure offset to 0.0:

• In the .con file, using SEASAVE or SBE Data Processing (for SBE 16plus, 16plus-IM, 19plus, 25, or 49).
• In the CTD's EEPROM, using the appropriate command in SEATERM (for SBE 16plus, 16plus-IM, 19plus, 37, 39, 39-IM, 49, 50, or 52-MP).

4. Collect pressure data from the instrument using SEASAVE or SEATERM (see instrument manual for details). If the instrument is not outputting data in decibars, convert the output to decibars.
5. Compare the instrument output to the reading from a good barometer placed at the same height as the pressure sensor. Calculate 
   offset (decibars) = barometer reading (converted to decibars) - instrument reading (decibars)
6. Enter calculated offset (positive or negative) in decibars:

• In the .con file, using SEASAVE or SBE Data Processing (for SBE 16plus, 16plus-IM, 19plus, 25, or 49).
• In the CTD's EEPROM, using the appropriate command in SEATERM (for SBE 16plus, 16plus-IM, 19plus, 37, 39, 39-IM, 49, 50, or 52-MP).

Note: For instruments that store calibration coefficients in EEPROM and also use a .con file (SBE 16plus, 16plus-IM, 19plus, and 49), set the pressure offset (Steps 3 and 6 above) in both the EEPROM and in the .con file.

Offset Correction Example:
Absolute pressure measured by a barometer is 1010.50 mbar. Pressure displayed from instrument is -2.5 dbars.
Convert barometer reading to dbars using the relationship:     mbar * 0.01 = dbars
Barometer reading = 1010.50 mbar *0.01 = 10.1050 dbars
Instrument’s internal calculations and/or our processing software output gage pressure, using an assumed value of 14.7 psi for atmospheric pressure. Convert instrument reading from gage to absolute by adding 14.7 psia to instrument output:
- 2.5 dbars + (14.7 psi * 0.689476 dbar/psia) = - 2.5 + 10.13 = 7.635 dbars
Offset = 10.1050 – 7.635 = + 2.47 dbar
Enter offset in .con file (if applicable) and in instrument EEPROM (if applicable).

Another source of offset error results from temperature-induced drifts. Because Druck sensors are carefully temperature compensated, errors from this source are small. Offset errors can be estimated for the conditions of your profile, and eliminated when post-processing the data in SBE Data Processing by the following procedure:

1. Immediately before beginning the profile, take a pre-cast in air pressure reading.
2. Immediately after ending the profile, take a post-cast in air pressure reading with the instrument at the same elevation and orientation. This reading reflects the change in the instrument temperature as a result of being submerged in the water during the profile.
3. Calculate the average of the pre- and post-cast readings. Enter the negative of the average value (in decibars) as the offset in the .con file.

Hysteresis Errors

Hysteresis is the term used to describe the failure of pressure sensors to repeat previous readings after exposure to other (typically higher) pressures. The Druck sensor employs a micro-machined silicon diaphragm into which the strain elements are implanted using semiconductor fabrication techniques. Unlike metal diaphragms, silicon's crystal structure is perfectly elastic, so the sensor is essentially free of pressure hysteresis.

Power Turn-On Transient

Druck pressure sensors exhibit virtually no power turn-on transient. The plot below, for a 3500-meter pressure sensor in an SBE 19plus SEACAT Profiler, is representative of the power turn-on transient for all pressure sensor ranges.

Thermal Transient

Pressure sensors exhibit a transient change in their output in response to changes in their environmental temperature, so the thermal transient resulting from submersion in water must be considered when deploying the instrument.

During calibration, the sensors are allowed to warm-up before calibration points are recorded. Similarly, for best depth accuracy the user should allow the CTD to warm-up for several minutes before beginning a profile; this can be part of the soak time in the surface water. Soaking also allows the CTD housing to approach thermal equilibrium (minimizing the housing's effect on measured temperature and conductivity) and permits a Beckman- or YSI-type dissolved oxygen sensor (if present) to polarize.

Last modified: 06-Apr-2007

Sea-Bird Home     Phone: 425-643-9866     Fax: 425-643-9954     E-mail: seabird@seabird.com